Duct Air Velocity
Use the Duct Air Velocity for hvac planning. Enter Airflow (CFM) and Duct diameter (in) to get a clear result, formula explanation, and practical checks.
Duct Air Velocity measurements
Enter your values, then calculate.
Result
How to calculate duct air velocity
This calculator finds air velocity inside a round duct from airflow (CFM) and duct diameter — an important check for noise, energy loss, and proper system design.
How the calculation works
Velocity (ft/min) = CFM ÷ (π × (Diameter/24)²). The duct's cross-sectional area is calculated from its radius (diameter/2, in feet since diameter is given in inches divided by 24), then CFM is divided by that area.
Example
400 CFM through a 10-inch diameter duct: Area = π × (10/24)² ≈ 0.545 sq ft; Velocity = 400 ÷ 0.545 ≈ 734 ft/min.
Frequently asked questions
How is Result calculated?
Result = [Airflow] ÷ (pi() × pow([Duct diameter] ÷ 24, 2)).
Is the Duct Air Velocity free to use?
Yes — every calculator on Simple Calculator Tools is free, runs in your browser, and does not require an account.
Duct Air Velocity
Velocity (ft/min) = CFM ÷ (π × (Diameter/24)²). The duct's cross-sectional area is calculated from its radius (diameter/2, in feet since diameter is given in inches divided by 24), then CFM is divided by that area.
Let's understand your duct air velocity result.
Calculate a result above and this guide will help you interpret it using this calculator's own formula and explanation.
Pro Tips for Duct Air Velocity
- Supply duct velocity is commonly targeted in the 700-900 ft/min range for residential systems to balance airflow with noise — check your specific design guideline.
- Velocity that's too high causes noise and pressure loss; velocity too low may indicate an oversized duct wasting material and space.
Common Duct Air Velocity Mistakes to Avoid
- Using the wrong diameter (rounding to a nominal size rather than actual duct ID) which introduces meaningful error into a velocity calculation.
When to Use This Calculator
This calculator finds air velocity inside a round duct from airflow (CFM) and duct diameter — an important check for noise, energy loss, and proper system design.